147 lines
4.0 KiB
C
147 lines
4.0 KiB
C
/*
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* Copyright (C) 2006 Michael Brown <mbrown@fensystems.co.uk>.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation; either version 2 of the
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* License, or any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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#include <stddef.h>
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#include <latch.h>
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#include <gpxe/list.h>
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#include <gpxe/process.h>
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#include <gpxe/init.h>
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#include <gpxe/retry.h>
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/** @file
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*
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* Retry timers
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*
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* A retry timer is a binary exponential backoff timer. It can be
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* used to build automatic retransmission into network protocols.
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*/
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/** Default timeout value */
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#define MIN_TIMEOUT ( TICKS_PER_SEC / 4 )
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/** Limit after which the timeout will be deemed permanent */
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#define MAX_TIMEOUT ( 10 * TICKS_PER_SEC )
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/* The theoretical minimum that the algorithm in stop_timer() can
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* adjust the timeout back down to is seven ticks, so set the minimum
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* timeout to at least that value for the sake of consistency.
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*/
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#if MIN_TIMEOUT < 7
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#undef MIN_TIMEOUT
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#define MIN_TIMEOUT 7
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#endif
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/** List of running timers */
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static LIST_HEAD ( timers );
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/**
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* Start timer
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*
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* @v timer Retry timer
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*
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* This starts the timer running with the current timeout value. If
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* stop_timer() is not called before the timer expires, the timer will
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* be stopped and the timer's callback function will be called.
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*/
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void start_timer ( struct retry_timer *timer ) {
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list_add ( &timer->list, &timers );
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timer->start = currticks();
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if ( timer->timeout < MIN_TIMEOUT )
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timer->timeout = MIN_TIMEOUT;
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}
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/**
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* Stop timer
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*
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* @v timer Retry timer
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*
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* This stops the timer and updates the timer's timeout value.
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*/
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void stop_timer ( struct retry_timer *timer ) {
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unsigned long old_timeout = timer->timeout;
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unsigned long runtime;
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list_del ( &timer->list );
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runtime = currticks() - timer->start;
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/* Update timer. Variables are:
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*
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* r = round-trip time estimate (i.e. runtime)
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* t = timeout value (i.e. timer->timeout)
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* s = smoothed round-trip time
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*
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* By choice, we set t = 4s, i.e. allow for four times the
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* normal round-trip time to pass before retransmitting.
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*
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* We want to smooth according to s := ( 7 s + r ) / 8
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*
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* Since we don't actually store s, this reduces to
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* t := ( 7 t / 8 ) + ( r / 2 )
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*
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*/
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timer->timeout -= ( timer->timeout >> 3 );
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timer->timeout += ( runtime >> 1 );
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if ( timer->timeout != old_timeout ) {
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DBG ( "Timer updated to %dms\n",
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( ( 1000 * timer->timeout ) / TICKS_PER_SEC ) );
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}
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}
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/**
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* Single-step the retry timer list
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*
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* @v process Retry timer process
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*/
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static void retry_step ( struct process *process ) {
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struct retry_timer *timer;
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struct retry_timer *tmp;
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unsigned long now = currticks();
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unsigned long used;
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int fail;
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list_for_each_entry_safe ( timer, tmp, &timers, list ) {
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used = ( now - timer->start );
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if ( used >= timer->timeout ) {
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/* Stop timer without performing RTT calculations */
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list_del ( &timer->list );
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/* Back off the timeout value */
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timer->timeout <<= 1;
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if ( ( fail = ( timer->timeout > MAX_TIMEOUT ) ) )
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timer->timeout = MAX_TIMEOUT;
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DBG ( "Timer backed off to %dms\n",
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( ( 1000 * timer->timeout ) / TICKS_PER_SEC ) );
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/* Call expiry callback */
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timer->expired ( timer, fail );
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}
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}
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schedule ( process );
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}
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/** Retry timer process */
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static struct process retry_process = {
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.step = retry_step,
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};
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/** Initialise the retry timer module */
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static void init_retry ( void ) {
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schedule ( &retry_process );
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}
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INIT_FN ( INIT_PROCESS, init_retry, NULL, NULL );
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